She was the traitor

Oa Star. Technology flourished on Oa Star, and its strange architecture bore the distinctive character of the cosmos. The enormous Green Lantern Central Power Battery towered above the surface, radiating a soft green glow. Green Lanterns from across the universe had been gathered together, assigned to patrol, conceal themselves, and guard various parts of Oa Star. In the space near Oa Star, a Green Lantern with a planet's consciousness lay hidden as well.